The overall objective of the proposed research is to attain a more complete understanding of mitosis in order to find ways to arrest cells specifically in mitosis. Human, hamster and human-hamster hybrid cells will be used as the main source of experimental material. Cell culture, cytological and biochemical methods will be used in the experiments. The specific objectives of the research include: 1. Further characterization of a temperature-sensitive mutant hamster cell line in which mitosis is arrested in metaphase at the non-permissive temperature. 2. Biochemical and ultrastructural analysis of the mutant to determine the specific gene product involved in the mutation. 3. Isolation of additional mutants blocked in mitosis for defining other gene products essential for the successful completion of mitosis. 4. Performance of genetic analysis with the mutants, including the utilization of human-mouse hybrid cells formed between the mutants and human cells. 5. Examination of the possibility that defects in mitosis could cause improper chromosome segregation and aneuploidy.